Many scientific, industrial, medical and military applications require the use of a high power laser beam. The required power is achieved by passing the laser beam at the output from the laser generating oscillator/preamplifier system through a laser power amplifier. However, since conventional laser amplifiers which utilize a laser gain medium having a generally rectangular cross section are not normally capable of achieving a gain in excess of 50 for reasons which will be discussed in greater detail hereinafter, large, complex, and thus relatively expensive oscillator/preamplifier systems are required. The low gain of the amplifiers is a particular problem since the efficiencies of the oscillators/preamplifiers are generally quite low and are inherently much less efficient than the power amplifier for several reasons.
For one thing, oscillators may be required to utilize optical components such as etalons and gratings which are lossy and also significantly decrease the laser extraction efficiency. The oscillator fields also build up from noise and hence the optical pulse length is always shorter than the discharge pulse length. Also, the oscillator gain length is relatively small. Therefore, mirror losses have a relatively larger impact on laser efficiency. Various controls, such as frequency and phase control, are also achieved in the oscillator, further reducing their efficiency.
For these reasons and others, the efficiency of an oscillator may be decreased by factors of 5-10 over that achievable in a carefully designed power amplifier. Therefore, it is desirable to increase the stage gain of the power amplifier as much as possible, preferably to values in the 100 to 500 range, since such increases can result in significant improvements in the size, weight and cost of the overall system.
One potential solution to achieving higher stage gain in a laser amplifier is discussed in a paper entitled "Expanding Beam Concept for Building Very Large Excimer Laser Amplifiers" by J. H. Jacob, M. Rokni, R. E. Klinkowstein and S. Singer (Appl. Phys. Lett. 48(5), Feb. 3, 1986). In this paper, it is postulated that substantially enhanced stage gains can be achieved from a laser amplifier by having the laser beam expand at a relatively small angle, for example, 2.degree.-5.degree., as the beam passes through the gain medium. This results in the normal stage gain of the amplifier, which is a product of the average gain g of the gain medium and the length L of the gain medium through which the beam traverses, and also results in area gain which is roughly proportional to A.sub.o /A.sub.i where A.sub.o and A.sub.i are the cross-sectional area at the output of the gain medium and the input of the gain medium respectively. Thus, by diverging the laser beam as it passes through the gain medium, improvements in the stage gain by a factor of 10 and more are theoretically achievable, resulting in an amplifier with a stage gain approaching 500, rather than an amplifier with a stage gain of no more than approxmately 50.
However, designing the electrodes for a diverging gain medium while still maintaining a constant electric field presents problems. The irregular configuration of the gain medium may also result in inefficient use of the gain medium, may create diffraction problems, and is generally not a preferred configuration. It is therefore desirable to be able to practice the expanding beam concept, and to achieve the improved gain which such concept provides, while still utilizing a rectangular gain medium.
While from the equation for stage gain (g L), it would appear that the stage gain of the gain medium could be increased to any desired level merely by increasing the length (L) of the medium, it has been found that amplified spontaneous emission (ASE) and parasitic modes cannot be controlled for a gain length product (g L) greater than about 4 which corresponds to a stage gain of 50. More particularly, when a laser amplifier is constructed using conventional rectangular geometry and an active medium that has an intrinsic non saturable absorption .alpha., the output power does not continually linearly increase as the medium length increases. Instead, the power increases linearly at first; however, when the absorption length product for such an amplifier exceeds 1, the output power stops increasing linearly with increasing length and instead the efficiency (power out/power in) of the laser decreases rapidly.
The reason for the reduction in efficiency arises from the relationship between the change in the effective laser gain along the medium and the non saturable absorption. More particularly, as the optical laser beam passes along the gain medium, the energy intensity or optical flux increases due to stimulated emission. As the flux increases, stimulated emission also increases and more rapidly decreases the population of inversion and hence the incremental gain eventually decreases. However, part of the absorption of the gain medium is non-saturable, and thus the overall gain which is the optical gain less the absorption decreases. Efficiency of extracting photons from the gain medium thus also decreases. Thus, the energy and power output from a laser constructed with conventional rectangular geometry cannot be increased by simply lengthening the gain medium without incurring a severe decrease in efficiency.
For the reasons indicated above, the maximum gain achievable with a normal rectangular gain medium is not greater than approximately 50. However, with the expanding gain medium, the length over which near-optimum power extraction can be obtained is significantly extended, the intensity increase caused by amplification being balanced by the intensity decrease caused by the area expansion. In this manner, the laser power extraction efficiency is maintained near optimum for the entire length of the amplifier. Further the stage gain in an expanding beam amplifier is approximately the product of the average gain times the length of the gain medium multiplied by the area gain. Typically, the product of the average gain times the length of the gain medium is limited to 50 because of ASE. However the multiplication of this number (50) and the area gain of approximately 10 results in stage gains that are an order of magnitude larger than the conventional amplifier.
However, in addition to the problem of designing electrodes for a long expanding beam amplifier, the length of the amplifier can make it difficult to design and cumbersome to use. It would therefore be desirable if the benefits of an expanding beam amplifier could be achieved with a more compact configuration.
It is therefore an object of this invention to provide an improved laser amplifier which is adapted to provide substantially enhanced gain over existing rectangular laser amplifiers while retaining a rectangular configuration.
A further object of this invention is to obtain the benefits of an expanding beam laser amplifier which may be several meters in length with a configuration which is more compact and thus less cumbersome to utilize.